This invention relates to fastening assemblies for fastening compressible insulation material onto a roof decking. A built up roof is made by providing a relatively thin sheet metal roof decking covering a framework of structural steel members. Insulation material such as mineral wool mat is then laid over the decking and secured in place by fastener assemblies. The insulation material and fastener assemblies are then covered by a waterproof membrane, for example a butyl rubber sheet or by a combination of roofing felt and a bitumastic sealing compound.
Recently it has been proposed that much thicker layers of insulation material are used in such roof construction. Recently it has been proposed that the thickness of the insulation be increased to a thickness within a range from 130 to 300 mm. One particular application of this is to provide a gravity fall on a flat roof deck by providing a graduated thickness of insulation material over the roof decking to convert a flat roof into one having a shallow pitch.